feat(s52): export Notifier public contract + sync extending.md B11

tests/test_notify.py

@@ -129,3 +129,23 @@ def test_config_fail_fast() -> None:
 
     notifier = build_notifier(webhook_cfg, allow_egress=True)
     assert isinstance(notifier, WebhookNotifier)
+
+
+def test_public_contract_exported() -> None:
+    """The declared B11 contract is public authoring API: importable from the package top and
+    listed in ``__all__`` (the brief-permitted export path — run.py's seam stays byte-intact)."""
+    import portfolio_optimiser
+
+    exported = (
+        "Notifier",
+        "ConsoleNotifier",
+        "FileNotifier",
+        "WebhookNotifier",
+        "build_notifier",
+        "NotifierConfig",
+        "NotifyError",
+        "NotifyRefused",
+    )
+    for name in exported:
+        assert name in portfolio_optimiser.__all__, f"{name} missing from __all__"
+        assert hasattr(portfolio_optimiser, name), f"{name} not importable from package top"